The requirements for high data transmission rates for wireless mobile communication appliances have recently increased to a major extent. In order to satisfy the requirements, various mobile radio standards have been developed which use high-efficiency modulation and access methods. In this case, the data to be transmitted is modulated both in the phase and the amplitude of the carrier signal. Depending on the chosen modulation method and the mobile radio standard, this combined amplitude and phase modulation can lead to high peak power levels in the output signal. These peak power levels occur in particular in base stations whose transmission signals have to cover a specific area. The average required output power for coverage of a base station which is combined to form a so-called communication cell requires power amplifiers with a high output power of up to 200 W.
Since the power amplifiers are the most costly components in a base station, it is expedient to operate them in the region of their maximum efficiency. This region is close to the 1-dB compression point on the characteristic of the power amplifier. The expression the 1-dB compression point means that region of the characteristic in which the characteristic of the power amplifier differs by 1 dB from an advantageous characteristic. The discrepancy is equivalent to distortion.
From this region of the characteristic, the power amplifiers which are used for the base station have a very highly non-linear dynamic response. The expression a “non-linear dynamic response” of an electrical network means that the network changes an input signal that is supplied to it by a non-linear factor, that is to say a non-proportional value. The output signal from the electrical network thus contains components which are not proportional to the input signal. The expression “dynamic” in this case means a memory effect. An input signal which is supplied to the electrical network at a first time influences the transmission response of the electrical network at a later time.
The non-linear transmission response of the power amplifier and of further switching elements in the base station leads together with an input signal, part of which has a high amplitude modulation level and thus high peak power levels, to intermodulation and distortion in the output signal. The output signal spectrum is thus broadened. Additional signal components in adjacent frequency ranges may, however, have a disturbing effect on signals there, and may lead to data errors. Modern mobile radio standards thus specify the power level which may be emitted into adjacent frequency ranges.
In order to reduce the distortion and the spectral broadening, it would be possible to appropriately design the elements causing distortion in a base station and to operate them in a linear region of their characteristic. This solution is not desirable because of the large amount of space consumed and current drawn, as well as the poor efficiency. A further method is to suitably-distort the input signal. Predistortion compensates for and corrects the distortion caused by the non-linear transmission response, thus reducing the spectral broadening and possible data errors in output signals.
A digital predistorter is frequently used for predistortion. This is in general formed by a highly complex, non-linear system which likewise takes account of memory effects. In order to provide a suitable digital predistortion, the non-linear transmission response of the radio-frequency amplifier can be identified based on input/output signal measurements. If this response is known, it can be approximated by means of models for the description of the digital predistorter. The results obtained can be transferred to the predistorter.
For this purpose, both the input signal and the output signal are normally sampled at a high sampling rate, in order to cover the regions outside the useful signal. These regions contain the signal components and distortion caused by the non-linear transmission response. If the overall signal bandwidth is Bout=N*Bin, where Bin represents the input signal bandwidth and N the highest order of non-linearity occurring in the power amplifier, the sampling rate is typically in the region of several hundred megasamples/second.
Thus, by way of example, the sampling rate is normally 250 MS/s for an input signal bandwidth of 20 MHz for four WCDMA/UMTS signals or WLAN signals, taking into account the non-linearity of the power amplifier up to the fifth order. The analog/digital converters required for sampling are correspondingly expensive.